There is a need to assist an emergency vehicle through stoplight intersections, for example, by detecting its presence and temporarily turning all traffic lights at the intersection red to facilitate clearing a safe path for the emergency vehicle. It is unduly expensive to equip each emergency vehicle with a special encoder for actuating the traffic lights.
Since it is traditional to equip each emergency vehicle with a siren either of an electronic or mechanical type, it is much more practical from an economic standpoint to provide suitable means for detecting the sound of the siren for the purpose of temporarily actuating the traffic lights to an emergency mode of operation.
A reliable siren detector must be able to discriminate against substantially all other noises, but experience shows that the siren exhibits an unusually difficult and unique amplitude and frequency pattern during its normal mode of operation. For example, the amplitude of the siren output, due to standing wave effects, varies by a factor of 20. The frequency of the siren signal can jump erratically from 800 cycles to 600 cycles due to standing wave effects. Analysis of the siren output shows that while in numerous situations it sweeps frequency over a prescribed range, there are many unexplained conditions wherein unexpected frequencies can occur at unpredictable times.
Other characteristics of sirens which have been observed and which prove troublesome to reliable detection include the fact that some of the mechanical sirens have cavity units interrelated to the tone generated by the siren wheel so as to make multiples, harmonics and the like in addition to the fundamental tones. Electronic sirens that are designed to duplicate the sound of mechanical sirens have sometimes included inductances in series with their frequency generating elements causing spurious harmonics to be generated and causing the frequency actually to leap from one value to another causing electronic sound detection equipment to pick up a confusing signal pattern.
The frequency pattern of a siren related to its harmonics, multiples, etc., has been found to change with distance of the siren to the microhphone. In addition, the siren sound reverberates between buildings, pavement and various objects and also passes through various standing wave conditions, depending upon the distance between the emergency vehicle and the microphone pick-up. The human ear does not notice many of these frequency jumps and multiples, so that a person, in determining whether a siren wail is in an increasing or decreasing frequency mode is generally unaware of the complexities of the actual sound from the siren.